The present invention is related to self-advancing support chocks useful in mines and various kind of workings to support the roof of galleries, more particularly in the neighbourhood of the mineral face behind the cutting machine.
Various types of self-advancing support chocks are known which, as a general rule, comprise a base element lying on the floor and supporting one or more props, each constituted by a ram with adjustable height and surmounted by a cap which, when the ram opens, applies itself to the roof of the gallery in order to support same. Among the various types of chocks, those called "shield or jib" chocks are more specially used to support the roof just behind the cutter in relatively low-quality grounds, with shales present and which give rise to important movements behind the opening made by the cutter, these movements tending to reduce considerably the distance between the floor and the roof of the gallery (convergence phenomenon). These chocks comprise a lever arm (shield or jib) articulated backwards on the base element and forwards on the cap, the ram (or rams) acting on this lever arm to push the cap against the roof, overhanging with respect to the front part of the chock, that is to say as close as possible to the mining face. The articulated lever arm acts as a multiplying device for the ram and thus permits an important vertical motion between the extreme positions of the cap (chock entirely opened and chock closed) to mitigate the great convergence between roof and floor. On the other hand, this arm also acts as a shield since it protects the chock from the rubbish which, due to the bad holding of the grounds, is under fragmented form.
In grounds having good cohesion, where the holding of the mining face is better and the convergence relatively smaller, shield chocks have a serious drawback. In such a kind of grounds, the roof collapses not in small pieces but as large and even very large slabs which, due to their weight, may crush the shield or at least damage it rapidly even if it is strongly stiffened. In order to obviate this drawback it is possible to break up the striking down, for example by blasting, but the working then becomes more costly and more dangerous.